Optical isolator employing a germanium-arsenic-selenium composition

ABSTRACT

A Faraday rotator for rotating a plane of polarization of polarized light, said Faraday rotator having an optical element comprising a rod which is comprised of an optically transmitting composition comprising from about 15 wt. % to about 50 wt. % germanium, from about 5 wt. % to about 20 wt. % arsenic, and from about 30 wt. % to about 80 wt. % selenium. This Faraday rotator is especially useful in an optical isolator for preventing feedback of transmitted light waves.

This application relates to optical isolators and Faraday rotators whichrotate the plane of polarization of polarized light. More particularly.this application relates to an optical element as part of the Faradayrotator. said optical element being made of a composition comprisinggermanium, arsenic, and selenium.

Optical isolators are devices which are used for operation with certainlasers or other light sources in the ultraviolet. visible. and infraredspectrums. An optical isolator permits the transmission of light in onedirection while blocking the reverse transmission of that light. Anoptical isolator is especially useful when used in conjunction with alaser in that it prevents optical feedback into the laser. This feedbackis analogous to electronic feedback.

The optical isolator works by rotating the plane of polarization ofpolarized light as the polarized light passes through a medium known asan optical element. The optical element is contained within a means forgenerating a magnetic field along the optical axis of rotation of saidoptical element. The optical element is made of a material having aVerdet constant. The Verdet constant is a measure of the ability of amaterial to rotate the plane of polarization of polarized light. Theoptical element and the means for generating a magnetic field along theoptical axis of rotation of the optical element is known as a FaradayRotator. An example of the means for generating the magnetic field forthe Faraday rotator is a donut-shaped magnet which has a hole forcontaining the optical element.

An optical isolator is comprised of an input polarizer. followed by theFaraday rotator, followed by an output polarizer. These three elementsare all contained in a body.

Light emitted from a source enters an input polarizer. The inputpolarizer causes light entering from one direction to be linearlypolarized. In this way, a plane of polarization of polarized light isformed. The polarized light then enters the optical element of theFaraday rotator, the optical element being surrounded by a means forgenerating a magnetic field such as a permanent magnet or anelectromagnet. The generation of a magnetic field along the optical axisof traveling light contained in the optical element rotates the plane ofpolarization of polarized light by a desired angle of rotation θ. Therotation of the plane of polarization may be clockwise orcounterclockwise. Light exiting the Faraday rotator, its plane ofpolarization having been rotated, enters an output polarizer. The planeof polarization in the output polarizer is parallel to the plane ofpolarization of the polarized light exiting the Faraday rotator. Thelight then exits the output polarizer and enters into a system and/orcarries out its intended use. A series of reflections of the light mayoccur, and some of the light may be reflected back to the originalsource. The reflected light, now traveling in the reverse direction,re-enters the output polarizer. The light, being of random polarization,again becomes polarized in a plane of polarization that is at an angleof rotation θ. The light then exits the output polarizer, and re-entersthe Faraday rotator, wherein the plane of polarization of there-polarized light is again rotated by the desired angle of rotation,thus resulting in a total angle of rotation 2θ. This again-rotatedpolarized light exits the Faraday rotator and re-enters the inputpolarizer in the reverse direction. If the total angle of rotation 2θequals 90°, extinction, or maximum isolation of the light which istraveling in the reverse direction, occurs. In this way, opticalfeedback is therefore prevented.

Applicant has invented an optical element for a Faraday rotator whichhas an improved ability to rotate the plane of polarization of polarizedlight. Applicant's invention, in one embodiment, is a Faraday rotatorwhich comprises an optical element in which is rotated a plane ofpolarization of polarized light along an optical axis. The opticalelement comprises a rod which comprises an optically transmittingcomposition comprising from about 15 wt. % to about 50 wt. % germanium,from about 5 wt. % to about 20 wt. % arsenic, and from about 30 wt. % toabout 80 wt. % selenium. Preferably, the composition comprises fromabout 30 wt. % to about 35 wt. % germanium, from about 10 wt. % to about15 wt. % arsenic, and from about 50 wt. % to about 60 wt. % selenium. Amost preferred composition for the optical element comprises Ge₃₃ As₁₂Se₅₅. The Faraday rotator also comprises a means for generating amagnetic field along the optical axis of said optical element, therebyrotating the plane of polarization of polarized light about the opticalaxis of the optical element. The means for generating a magnetic fieldmay be a permanent magnet or an electromagnet. The magnet may be of anyshape. In one embodiment, the magnet is in the shape of a donut having ahole in which the optical element is contained. The optical element maybe contained within a tube made of a non-magnetic material such asbrass, said tube being disposed between the optical element and themagnet.

Applicant's invention also includes an optical isolator having an inputpolarizer for polarizing beams or waves of light, a Faraday rotator ofthe embodiment mentioned above, and an output polarizer. Applicant'sinvention is also directed to a method of rotating the plane ofpolarization of polarized light using the optical isolator or Faradayrotator described above.

FIG. 1 is a break-away diagram of an embodiment of a Faraday rotator inaccordance with the present invention; and

FIG. 2 is a block diagram of the operation of an optical isolator inaccordance with an embodiment of the present invention.

Referring now to the drawings, a Faraday rotator in accordance with oneembodiment of the present invention comprises an optical element and amagnet. In the embodiment shown, the optical element is in the form of arod. The rod can be surrounded by a non-magnetic material such as brass.The magnet, in the embodiment shown, is in the form of a donut whichcompletely surrounds the optical element, but magnets of other shapesmay be used as long as a magnetic field can be generated along theoptical axis, along which polarized light travels within the opticalelement. The magnet may be a permanent magnet or an electromagnet.

The optical element comprises a rod which is comprised of an opticallytransmitting composition comprising from about 15 wt. % to about 50 wt.% germanium, from about 5 wt. % to about 20 wt. % arsenic, and fromabout 30 wt. % to about 80 wt. % selenium. Preferably, the compositioncomprises from about 30 wt. % to about 35 wt. % germanium, from about 10wt. % to about 15 wt. % arsenic, and from about 50 wt. % to about 60 wt.% selenium. Most preferably, this composition comprises Ge₃₃ As₁₂ Se₅₅.This type of optical element has improved ability to rotate the plane ofpolarization of polarized light. The composition of the formula Ge₃₃As₁₂ Se₅₅ has a Verdet constant of +0.160 minutes/cm-oersted at awavelength of 1064 nm. This material, therefore, is in what is known asthe class of diamagnetic materials. A preferred embodiment of a Ge₃₃As₁₂ Se₅₅ composition is a product known as AMTIR^(R) -1, a product ofAmorphous Materials Incorporated, of Garland, Texas.

In FIG. 1 are shown the directions of light waves or beams which travelthrough the optical element. Polarized light travels in one directionfrom the light source along an optical axis, and reflected polarizedlight travels in a direction opposite that of the polarized lighttraveling from the light source, along an optical axis. The magneticfield generated by the donut-shaped magnet is in a direction toward oraway from the light source. The magnetic field generated by the magnetalong the optical axis enables the plane of polarization of polarizedlight to be rotated by a desired angle of rotation θ. This rotation maybe clockwise or counterclockwise.

Referring now to FIG. 2, it will now be seen how an embodiment of anoptical isolator in accordance with embodiment of the present inventionserves to prevent feedback of light waves.

Light from a source, traveling in the forward mode, enters an inputpolarizer which polarizes the light by causing the light to be linearlypolarized, thus creating a plane of polarization of polarized light. Thepolarized light exits the input polarizer and enters the Faradayrotator.

The polarized light enters the optically transmitting optical elementcomprised of the composition of germanium, arsenic, and selenium asdescribed above, of the Faraday rotator along an optical axis. Theoptical element, by virtue of the generation by a magnet of a magneticfield along the optical axis, rotates the plane of polarization ofpolarized light by a desired angle of rotation θ. This rotation may beclockwise or counterclockwise. In the embodiment depicted in FIG. 2, theangle of rotation θ is 45°. The polarized light, thus having had itsplane of polarization rotated by 45°, exits the Faraday rotator andenters the output polarizer or analyzer. The output polarizer has aplane of polarization parallel to that of the polarized light which hasexited the Faraday rotator. Light which exits the output polarizer thenenters into a system and/or begins its intended use. A series ofreflections may occur, and some of the light is transmitted back towardthe light source in the reverse mode in random planes of polarization.

In the reverse mode, the reflected polarized light, which was travelingin random planes of polarization, reenters the output polarizer in whicha plane of polarization having an angle of rotation of θ, or 45°, isformed for the reflected polarized light. The polarized light then exitsthe output polarizer and reenters the optical element of the Faradayrotator. The polarized light, which was linearly polarized at an angleof 45° in the output polarizer while traveling the reverse mode, now hasits plane of polarization rotated another 45° while traveling in thereverse mode within the optical element, thus making the total angle ofrotation of 2θ, or 90°. The polarized light, which has now had its planeof polarization rotated by a total angle of rotation of 90°, exits theFaraday rotator and reenters the input polarizer.

In the input polarizer, the reflected light which has traveled in thereverse mode, with its plane of polarization having been rotated 90°,becomes extinct. Maximum isolation of this light now occurs, thuspreventing optical feedback.

In some embodiments the plane of polarization of the polarized light maynot be rotated by the optical element in the Faraday rotator at an angleof 45° during each pass of the light through the optical element of theFaraday rotator, said optical element being comprised of the compositioncomprising germanium, arsenic, and selenium as described above. In thesecases, the polarized light traveling in the reverse mode will have aplane of polarization that has not been rotated for a total angle ofrotation that is 90°. Because of this, there will be some feedback tothe light source but not as much as if there were no optical isolatorpresent next to the light source.

It is to be understood the scope of the present invention is not to belimited to the specific embodiments described above. The invention maybe practiced other than as particularly described and still be withinthe scope of the accompanying claims.

What is claimed is:
 1. A Faraday rotor comprising an optical element inwhich is rotated a plane of polarization of polarized light along anoptical axis, said optical element comprising a rod comprising anoptically transmitting composition comprising from about 15 wt. % toabout 50 wt. % germanium, from about 5 wt. % to about 20 wt. % arsenic,and from about 30 wt. % to about 80 wt. % selenium; and means forgenerating a magnetic field along the optical axis of said opticalelement, thereby rotating said plane of polarization of polarized lightin said optical element.
 2. The Faraday rotator of claim 1 wherein saidoptically transmitting composition comprises from about 30 wt. % toabout 35 wt. % germanium, from about 10 wt. % to about 15 wt. % arsenic,and from about 50 wt. % to about 60 wt. % selenium.
 3. The Faradayrotator of claim 2 wherein said optically transmitting compositioncomprises Ge₃₃ As₁₂ Se₅₅.
 4. The Faraday rotator of claim 1 where saidmeans for generating a magnetic field comprises a permanent magnet. 5.The Faraday rotator of claim 1 wherein said means for generating amagnetic field comprises an electromagnet.
 6. An optical isolatorcomprising:an input polarizer for polarizing beams or waves of light; aFaraday rotator for rotating a plane of polarization of polarized light,said Faraday rotator comprising: an optical element in which is rotateda plane of polarization of polarized light along an optical axis, saidoptical element comprising a rod comprising an optically transmittingcomposition comprising from about 15 wt. % to about 50 wt. % germanium,from about 5 wt. % to about 20 wt. % arsenic, and from about 30 wt. % toabout 80 wt. % selenium; and means for generating a magnetic field alongthe optical axis of said optical element, thereby rotating said plane ofpolarization of polarized light in said optical element; and an outputpolarizer.
 7. The optical isolator of claim 6 wherein said opticallytransmitting composition comprises from about 30 wt. % to about 35 wt. %germanium, from about 10 wt. % to about 15 wt. % arsenic, and from about50 wt. % to about 60 wt. % selenium.
 8. The optical isolator of claim 7wherein said optically transmitting composition comprises Ge₃₃ As₁₂Se₅₅.
 9. The optical isolator of claim 6 wherein said means forgenerating a magnetic field comprises a permanent magnet.
 10. Theoptical isolator of claim 6 wherein said means for generating a magneticfield comprises an electromagnet.
 11. An optical element comprising arod comprising an optically transmitting composition comprising fromabout 15 wt. % to about 50 wt. % germanium, from about 5 wt. % to about20 wt. % arsenic and from about 30 wt. % to about 80 wt. % selenium. 12.The optical element of claim 11 wherein said optically transmittingcomposition comprises from about 30 wt. % to about 35 wt. % germanium,from about 10 wt. % to about 15 wt. % arsenic, and from about 50 wt. %to about 60 wt. % selenium.
 13. The optical element of claim 12 whereinsaid optically transmitting material comprises Ge₃₃ As₁₂ Se₅₅.